Surveillance system

ABSTRACT

A surveillance system having a primary station from which signals are issued upon a coaxial cable to one or more surveillance stations. Each surveillance station selectively can impose an impedance discontinuity upon the cable which gives rise to a characteristic signal reflection and a measurable interval from the signal issuance to its reflected return to the primary station. This interval is indicative of the separation of the primary and surveillance stations. Various forms of surveillance station actuatory are contemplated for introducing the impedance discontinuities at the station in response to various actuating conditions. The system also locates breaks or other impedance discontinuities in the cable at other than surveillance stations. Where discontinuities are controllable, as by surveillance station means, they can be varied from time to time to convey supplemental information to the primary station and they can be imposed for brief intervals to enable the continued monitoring of other surveillance stations beyond the activated station during the periods that activated station is not imposing a discontinuity.

United States Patent [191 Faber et al.

[ 1 May8, 1973 [54] SURVEILLANCE SYSTEM [76] Inventors: Robert Charles Faber, 2549 Goddard Road, Toledo, Ohio 43606; Stephen B. Miller, 113-27th Street, Apt. B, Manhattan Beach, Calif. 90266 [22] Filed: Aug. 19, 1970 [2]] Appl. No.: 65,184

[52] US. Cl. ..340/408, 324/52, 340/150, 340/170, 340/181, 340/207, 340/216, 340/288 [51] Int. Cl ..G08b 5/22 [58] Field of Search ..340/408, 150, 258 R, 340/258 C, 203, 312,181, 215,170, 216, 416, 207 R, 207 P, 183; 324/52; 179/1753, 5 R

[56] References Cited UNITED STATES PATENTS 2,345,932 4/1944 Gould ..179/175.3 X 3,462,756 8/1969 Mills ..340/216 X 2,769,167 10/1956 Gloess ....340/170 X 2,860,324 11/1958 Berger et al. ..340/170 X 3,214,734 10/1965 Whitehead ..340/150 X 3,493,949 2/1970 Servos et al. ..324/52 X FOREIGN PATENTS OR APPLICATIONS 260,386 11/1963 Australia ..340/258 I I l DISCRIMINATOR P FREQUENCY AND/OR PHASE INDICATOR/CONTROL 26 GATE PHASE 7 CONTROL SEN SOR INDICATOR CONTROL Primary ExaminerDavid L. Trafton Attorney-Wilson & Fraser [57] ABSTRACT A surveillance system having a primary station from which signals are issued upon a coaxial cable to one or more surveillance stations. Each surveillance station selectively can impose an impedance discontinuity upon the cable which gives rise to a characteristic signal reflection and a measurable interval from the signal issuance to its reflected return to the primary station. This interval is indicative of the separation of the primary and surveillance stations. Various forms of surveillance station actuatory are contemplated for introducing the impedance discontinuities at the station in response to various actuating conditions. The system also locates breaks or other impedance discontinuities in the cable at other than surveillance stations. Where discontinuities are controllable, as by surveillance station means, they can be varied from time to time to convey supplemental information to the primary station and they can be imposed for brief intervals to enable the continued monitoring of other surveillance stations beyond the activated station dur ing the periods that activated station is not imposing a discontinuity.

17 Claims, 5 Drawing Figures F l4 l5 SURVEILLANCE STATION *1 DISCRWIINATOR FREQUENCY f42 AND/OR PHASE INDICATOR/CONTROL Pmimgnmv ems FIG.

TIVA

SURVEILLANCE STATION DISCRIMINATOR FREQUENCY AN D/OR PHASE NDICATOR/CONTROL INDICATOR SURVEILLANCE STATION *2 M X b I n t [L r 4 3 H 8 L E 3 00 C T. H W KN 5 7 2 WC EN 4 2 R E O lllll IIMS fi m S 3 m ER I 1 I Mm IF P O R C m H m m M NM 2 u m a SE 3 5 G 3 O DISCRIMINATOR FREQUENCY AN D /OR PHASE NDlCTOR/CONTROL FIG. 5

INVENTORS ROBERT CHARLES FABER STEPHEN B. MILLER ATTORNEYS SURVEILLANCE SYSTEM BACKGROUND OF TI-IE INVENTION 1 Field of the Invention This invention relates to surveillance systems and more particularly to systems having a primary station which applies signals to a transmission line and senses reflections of those signals as an indication of a condition along the line and the location of that condition.

2. Description of the Prior Art Surveillance systems of various forms have been employed for conveying information from locations in buildings, plant complexes, and municipalities. These have included coded, noncoded, supervised and nonsupervised systems wherein a coded system has surveillance stations providing a distinctive signal upon activation to identify the activated surveillance station for the supervisory or primary station, and wherein a supervised system indicates a fault which impairs the system or precludes transmission of signals. In general,

these systems can be arranged in the combinations, as coded-nonsupervised, coded-supervised, noncodednonsupervised, and noncoded-supervised, by providing separate coding and supervising equipment. Coding frequently requires a multiplicity of conductive paths and or complex coding equipment at the surveillance station for controlling and in some instances, transmitting signals. Usually, where a plurality of surveillance stations control signals on a common transmission circuit, the operation of one station precludes the monitoring of any additional stations on that circuit. A single information transmission state, as on and of is ordinarily controlled from each surveillance station.

Surveillance systems having means for opening and closing contacts to complete or interrupt circuits are subject to contact resistance variations which can introduce marginal operation. Further, voltage drops along the conductors, where long runs are required, can degrade operation and reliability where signals are issued from the surveillance stations.

Many of the prior systems have employed direct current or low-frequency alternating current as the signal energy. They have been arranged with manually operable signaling devices at their surveillance stations, as for monitoring the rounds of a watchman, or for transmitting a help call" from a watchman, with thermostatic detectors, for fire detection, with smoke detectors employing photoelectric and ionization sensors, and with intrusion detectors employing ultrasonic and infared sensors, to name a few. These automatic devices usually require local power at the surveillance station for operation and when coded require pPwer for coding.

Intrusion sensing systems have been arranged with precisely and uniformly related conductors providing a transmission line having a uniform characteristic impedance. In these systems a high frequency signal is transmitted down the line and impedance perterbations caused by objects in the vicinity of the line cause a portion of the signal to be reflected back along the line to the source. The return signal is used to indicate the presence of the object and its approximate location along the line. Such lines are extremely expensive to install and maintain. They tend to have unstable charac teristics and thus issue false alarms. They are sensitive to environmental changes and issue false alarms in response to changes such as rain, snow and high humidity. When employed with oscilloscope displays, these systems require a skilled operator for interpretation of signals.

SUMMARY OF THE INVENTION The present invention is a surveillance system employing a sharp wave front signal applied to a coaxial cable system having a substantially uniform impedance along its length and a detecting means for ascertaining the phase and reflection time of the signal reflection of that sharp wave front. Surveillance stations can be serially connected into the line so that signals can be transmitted to the primary station from any of the surveillance stations. A coaxial cable has low loss and a uniform impedance characteristic along its length, its characteristic impedance, and is self-shielding so that it is unaffected by the usual environmental changes.

Switches can be located in the coaxial cable transmission line so they impose little or no impedance discontinuity on the line when they are not actuated to intentionally introduce a discontinuity. These switches are arranged for actuation by any of the usual actuating conditions effective at conventional surveillance stations including manual means, by heat, by light, by smoke or vapor, and by auxiliary detecting systems of the ultrasonic or radiant electromagnetic energy types.

The system can be effective at a station or at points spaced from a station where the transmission line is broken, crushed or otherwise disrupted. Since it responds to an open circuit disruption, with a reflected signal shifted in phase, e.g. to the opposite polarity, from that of a short circuit disruption, the switches can be arranged to impose both types of discontinuity in accordance with the coding established for the condition to be indicated at the station. In the case of short circuits for the sharp wave front signal, a capacitance is employed to provide an effective open circuit where the coaxial cable is also employed as a direct current or low frequency conductor, as where power is transmitted over the conductors of the cable from the primary station to the surveillance station. Power transmission can be continuous or in response to a sensed condition as where an acknowledging signal is desired.

Advantageously, the surveillance stations are arranged to intermittently present the impedance discontinuity. In this manner the remainder of the system can be monitored when the discontinuity is removed. Such a system requires signal pulses to be coincident with the signalling discontinuity as by having a pulse frequency from the primary station which is great enough to insure at least two pulses during the interval the discontinuity is imposed.

DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic representation of the system employed as an alarm system for a plant complex comprising a main structure and an outbuilding in which a surveillance station is located at each of a plurality of locations;

FIG. 2 is a block diagram of one embodiment of a surveillance system according to this invention; and

FIGS. 3, 4 and 5 are schematic representations of surveillance station switches for the coaxial cable illustrating respectively a shorting switch, a circuit opening switch, and a combined shorting switch and circuit opening switch.

DESCRIPTION OF THE PREFERRED EMBODIMENTS A representation of a typical utilization of a surveillance system according to this invention is disclosed in FIG. 1 where the system is installed at a plant site including a main building 11 and an outbuilding 12. A primary station 13 is located at some convenient point in the plant complex and affords the means to generate, receive and analyze signals and issue indications of the region of the plant in which an activating condition exists and, in some instances the nature of that condition. Coaxial cable 14 extends from the primary station to a plurality of surveillance stations 15 distributed as desired throughout the plant site. Activating means are provided at each surveillance station to impose selectively in response to an activating condition an impedance discontinuity on the normally constant impedance, the characteristic impedance, of the coaxial cable. Such an impedance discontinuity, when subjected to a sharp wave front signal transmitted from the primary station, produces a reflection of that signal to the primary station. The signal monitoring apparatus of the primary station 13 senses the distance of the discontinuity from the primary station and the phase of the reflection. This is employed as the means of conveying information from surveillance stations, however, such reflections are also sensed and the origin is located when the cable is damaged at other than surveillance stations as by a mechanical break, fire, or crushing between such stations.

Primary station 13 includes a signal source, means to couple signals to coaxial cable 14 and means for interpretation of the reflected signals from coaxial cable 14. A pulse or step signal generator 16 produces a signal having a sharp front and thus a signal of an effective high frequency characteristic insofar as its relationship to the circuits and transmission line in which it is transmitted. A pulse generator 16 is controlled by an oscillator 17 through coincidence gate 18 for timing purposes.

The sharp wave front pulse issuing from generator 16 is passed by switch 19 when in the sensing condition, to coaxial line 14 and along that line until it encounters a significant impedance discontinuity from the characteristic impedance of the line to cause a significant reflection of the signal. A gated signal on lead 21 from gate 18 to generator 16 results in a sharp wave front signal on line 22. The signal on line 22 has no immediate effect on the circuits coupled through coaxial T connectors 23 and 24 since delay 25 prevents any signal being effective on gate control 26 or phase sensor 27 for the first pulse of oscillator 17. This prevents the transmitted sharp wavefront signal from having any effect on gate control 26 and phase sensor 27. When the transmitted signal from generator 16 decays, the signal from delay 25 activates gate control 26 and phase sensor 27 so that gating of oscillator pulses through gate 28 is enabled by gate control 26 until a sharp wave front reflection is returned from line 14 through switch 19 to T23 and gate control 26. The reflected signal, when of significant magnitude causes gate control 26 to inhibit gate 28.

In the interval gate 28 is enabled, the pulses issued by oscillator 17 are passed to counter 29 and accumulated therein as a count representative of the transit time of the sharp wave front from primary station 13 to a reflecting impedance discontinuity on the line and return. More particularly, where the velocity of propagation of the sharp wave front signal on the coaxial cable is percent of the velocity of propagation of signals in free space, a resolution of the position of the reflecting discontinuity along the line of approximately 0.5 meter per oscillator pulse is achieved with an oscillator frequency of 350 megacycles. Position measurements therefore are ascertained by a pulse counter 29 cooperating with a decoder 31 which assigns specific counts or ranges of counts to surveillance stations or zones along the coaxial cable length.

The reflected signal is passed by T24 to provide further information as to the nature of the impedance discontinuity. An open circuit termination reflects a signal which is shifted in phase from a short circuit termination. Phase sensor 27 is fed signals from T24 and identifies their phase. It issues an indication of the phase to decoder 31 whereby the coder is able to identify the position and the type of discontinuity. The utilization of such information will be discussed below.

Monitoring of the system from primary station 13 can be on a one shot basis, by actuating the signal generator 16 to issue a single sharp wave front signal. Such a signal can be issued periodically provided this is done with an interval at least as great as that necessary to complete the decoding of a signal transmitted and reflected, the maximum distance available in the system as to the open circuit termination of coaxial cable 14 at 32. In the preferred arrangement, the system recycles itself on return of the reflected sharp wave front signal. When gate control 26 receives the reflection and issues an inhibiting signal to gate 28 to terminate the pulse count, it also issues a signal on lead 33 to enable gate 18 to pass the next pulse from oscillator 17 to lead 21 and the triggering input for signal generator 16. Thus, generator 16 initiates a sharp wave front signal immediately following the preceding signal reflection.

Decoder 31 actuates an indicator control 34 in response to the count and phase decoding to issue several useful signals. Assume that the actuating mechanism at the surveillance stations 15 is a manually actuated fire alarm including a manual switch 44 for imposing a short circuit to the high frequency of the sharp wave front signal to indicate the presence of a fire, and a manual switch 45 for imposing an open circuit to the high frequency of the sharp wave front to indicate a reset" or all clear. A fire signal at surveillance station number 1 by a short circuit would result in a predetermined low count in counter 29 and a predetermined phase indication from indicator 27 to set decoder 31 and cause it, through control 34, to actuate indicator 35. Indicator 35 can be an annunciator (not shown) which displays as signal Fire at station No. 1 and/or an audible signal or equivalent device for issuing an immediate alarm. Similarly, a second annunciator or a reset for the first can be incorporated in indicator 35 so that upon actuation of the reset switch at station No. I an all clear is indicated or the Fire at station No. l annunciator is reset.

The signalling from a surveillance station subsequent to a first signal can be made effective by merely transferring the type of termination at that station. However, it is frequently desirable to continue to monitor the entire system and particularly those portions beyond the actuated surveillance station. In order to enable the primary station to transmit signals beyond the actuated surveillance station and to receive reflections from beyond that station, the surveillance station switches for transferring between the characteristic impedance of the line and a terminating impedance are arranged to introduce the terminating impedance for an interval sufficient to actuate the monitoring equipment. Advantageously, this interval should be at least two monitoring pulse cycles to insure operation of the monitoring equipment. Thereafter, the switch should return to its deactuated state to again provide a characteristic impedance beyond which signals can be transmitted. Accordingly, several or even all stations can indicate the conditions which the system is designed to supervise.

Another function of the indicator control is that of controlling indicators 36 and 37 spaced from the primary station. An elemental form of such indication is to actuate all such indicators distributed throughout'the plant. More refined systems selectively actuate such indicators in any desired response pattern, as an individual indicator associated with the zone of the plant site in which the surveillance station has been actuated, or as such an indicator in conjunction with those in zones of the plant site which are related to the zone of the activated indicator.

Indicator control 34 activates remote indicators such as 36 and 37 by transmitting activating signals of low frequency or direct current along the conductors of coaxial cable. In one embodiment controlling signals are passed on 38 from indicator control 34 to switch 19 to transfer the coupling of line 14 from signal generator 16 to a low frequency or direct current source integral with 34. This signal can be passed along the line 14 to the various indicators 36 and 37 through coaxial T connectors 39, arranged to match the characteristic impedance of line 14 and thereby avoid undesired reflections, and chokes 41, arranged to prevent the transmission of the high frequency signals of the sharp wave front pulses from generator 16 and to avoid introducing reflecting impedances to those signals.

Where selective actuation of remote indicators is desired, the indicator control 34 is provided with selected ringing controls wherein the indicator signals it issues are polarized, as positive or negative pulses, and are of coded frequencies, frequencies sufficiently spaced from indicator zone to indicator zone to be positively segregated and yet of a sufficiently low frequency to have transmission characteristics distinct from the sharp front wave to be reflected in the monitoring function of the system. Such selective ringing signals are well known in telephone practice.

Selective actuation of the remote indicators require discriminators 42 which can be polarity and/or frequency sensitive devices. Band pass filters having polarizing rectifiers (not shown) and vibrating reed contacts (not shown) are two forms of such discriminators for passing indicator actuating signals to indicators 36 and 37.

When switch 19 is arranged to decouple signal generator 16 from line 14 and couple indicator. control 34 to line 14, and when it is desired to continue monitoring of the surveillance stations from the primary station, the switch is intermittently transferred between the generator 16 and control 34 by control 34. Thus, the remote indicators can be actuated while coupled to control 34 and can be deactivated while the system is monitored.

Continuous power can be supplied on the line 14 to the surveillance stations and/or the remote indicators. Thus, where the activating means at each surveillance station requires continuous energization, the energy can be supplied over the line 14 continuously. Where such continuous energization is imposed, short circuit terminations for the reflected sharp wave front signal are provided with a capacitance 43 as shown in FIGS. 3 and 5. FIGS. 3, 4 and 5 represent surveillance station switches, but not the actuating means therefore, as the circuit within the dashed rectangles 44, 45 and 46 having cable connections 47 and arranged in the condition shown to impose no discontinuity in the line. When the switch 48 of FIG. 3 is closed, switch unit 44 presents a short circuit termination of the line 14 at the surveillance station of the switch sofar as the sharp wave front signal is concerned. However, capacitance 43 maintains the high frequency short circuit as an open circuit to the low frequency signals transmitted on coaxial cable 14 as operating power for the surveillance stations or actuating signals for the remote indicators or both.

FIG. 4 represents switch unit 45 providing means to introduce an open circuit termination for cable 14 at a surveillance station by opening switch 49. Ordinarily, the low frequency signals of the system can tolerate momentary interruptions as would be introduced where the short circuit termination was applied for only a brief interval. If the switch unit is of a nature to maintain an open circuit for substantial intervals, a low frequency shunt (not shown) can be placed around the switch 49 as a choke.

FIG. 5 shows a switch unit for a surveillance station offering means to introduce a short circuit termination to the pulses to be reflected by closing switch 51 and means to introduce an open circuit termination to those pulses by switch 52.

The above set forth surveillance system is typical of the type contemplated according to this invention utilizing a coaxial cable transmission line on which the transmission and reflection of sharp front waves can be transmitted between a primary station and a surveillance station to convey useful information. It should be understood that many variations of the system can be introduced in that it lends itself to various forms of monitoring, even including an oscilloscope A-scope" display of the indicated spacing of the impedance discontinuity from the primary station; recording type indicators, and controls which activate equipment such as sprinkler systems.

Further, the forms of activating means for surveillance stations can be highly refined and automatically responsive to environmental conditions. Accordingly, the present disclosure is to be read as illustrative of the invention and not in a limiting sense.

What is claimed is:

1. A surveillance system comprising a surveillance station; a primary station; a coaxial cable extending from said primary station to said surveillance station and having a characteristic impedance; a termination for said coaxial cable at its end; means for issuing a sharp wave front signal on said coaxial cable from said primary station; means at said primary station for sensing a reflection of said signal from said end termination; means at said surveillance station responsive to a predetermined actuating condition at said station for selectively imposing a predetermined impedance discontinuity to the sharp wave front signal on said coaxial cable; means at said primary station for sensing a reflection of said signal from an imposed impedance discontinuity on said coaxial cable intermediate said primary station and said termination; and an indicator actuated by the response of said last mentioned means.

2. A combination according to claim 1 wherein said sensing means includes means for ascertaining the time between the issuance of the sharp wave front signal and the reflection of said sharp wave front.

3. A combination according to claim 2 including means to translate the measured time to a region along the length of said coaxial cable.

4. A combination according to claim 1 wherein said sensing means includes means for ascertaining the phase of the reflected sharp wave front signal.

5. A combination according to claim 1 wherein said surveillance station includes means for imposing said impedance discontinuity for a brief interval in response to an activating condition at said station and thereafter removing said discontinuity even though the activating condition persists whereby signals can be reflected from beyond said activated station to said primary station.

6. A combination according to claim 1 wherein said surveillance station includes means for imposing a short circuit to said sharp wave front signal in response to an activating condition.

7. A combination according to claim 1 wherein said surveillance station includes means for imposing an open circuit to said sharp wave front signal in response to an activating condition.

8. A combination according to claim 1 wherein said coaxial cable provides a circuit for very low frequency signals between said primary station and said end termination and wherein said surveillance station includes means to prevent a discontinuity in said low frequency circuit when said actuating condition imposes an impedance discontinuity to the sharp wave front signal.

9. A combination according to claim 8 wherein said surveillance station includes means to selectively impose a short circuit as an impedance discontinuity to the sharp wave front signal and wherein a capacitance of a value to present an open circuit to said low frequency circuit and a low impedance to said sharp wave front signal is connected in series with said means to impose a short circuit.

10. A combination according to claim 1 including means at said primary station responsive to a reflection of said signal for actuating said means for issuing the sharp wave front signal to issue such a signal.

1 l. A combination according to claim 1 wherein said primary station includes a clocking oscillator; a gate control coupled to said coaxial cable and responsive following issuance of a signal from said signal issuing means and until a reflection of said signal is received from said cable; a gate passing clocking pluses from said oscillator when said gate control is responsive; and a clock pulse counter for counting pulses from said gate.

12. A combination according to claim 11 including means issuing an actuating signal to said means for issuing a sharp wave front signal; a delay coupled to said means issuing the actuating signal; and inhibiting means for said gate control which are rendered ineffective by a signal from said delay coupled to said delay.

13. A combination according to claim 1 wherein said primary station includes a phase sensor coupled to said coaxial cable; and means rendering said phase sensor responsive to reflections of the sharp wave front signal following the issuance of the sharp wave front signal by said issuing means.

14. A combination according to claim 3 including means for ascertaining the phase of the reflected sharp wave front signal.

15. A combination according to claim 1 wherein said indicator is coupled to said primary station by said coaxial cable and is remote from said station.

16. A combination according to claim 1 wherein said indicator is a plurality of indicating means each coupled to said primary station by said coaxial cable and spaced from said primary station and each other.

17. A combination according to claim 16 wherein said sensing means includes means for ascertaining the time between the issuance of the sharp wave front signal and the reflection of said signal; wherein said primary station includes an indicator control responsive to predetermined reflection time intervals defined by said ascertaining means to issue predetermined different coded signals according to said reflection time intervals; and wherein each of a piurality of said indicating means have a decoding actuating means responsive to given predetermined coded signals from said indicator control. 

1. A surveillance system comprising a surveillance station; a primary station; a coaxial cable extending from said primary station to said surveillance station and having a characteristic impedance; a termination for said coaxial cable at its end; means for issuing a sharp wave front signal on said coaxial cable from said primary station; means at said primary station for sensing a reflection of said signal from said end termination; means at said surveillance station responsive to a predetermined actuating condition at said station for selectively imposing a predetermined impedance discontinuity to the sharp wave front signal on said coaxial cable; means at said primary station for sensing a reflection of said signal from an imposed impedance discontinuity on said coaxial cable intermediate said primary station and said termination; and an indicator actuated by the response of said last mentioned means.
 2. A combination according to claim 1 wherein said sensing means includes means for ascertaining the time between the issuance of the sharp wave front signal and the reflection of said sharp wave front.
 3. A combination according to claim 2 including means to translate the measured time to a region along the length of said coaxial cable.
 4. A combination according to claim 1 wherein said sensing means includes means for ascertaining the phase of the reflected sharp wave front signal.
 5. A combination according to claim 1 wherein said surveillance station includes means for imposing said impedance discontinuity for a brief interval in response to an activating condition at said station and thereafter removing said discontinuity even though the activating condition persists whereby signals can be reflected from beyond said activated station to said primary station.
 6. A combination according to claim 1 wherein said surveillance station includes means for imposing a short circuit to said sharP wave front signal in response to an activating condition.
 7. A combination according to claim 1 wherein said surveillance station includes means for imposing an open circuit to said sharp wave front signal in response to an activating condition.
 8. A combination according to claim 1 wherein said coaxial cable provides a circuit for very low frequency signals between said primary station and said end termination and wherein said surveillance station includes means to prevent a discontinuity in said low frequency circuit when said actuating condition imposes an impedance discontinuity to the sharp wave front signal.
 9. A combination according to claim 8 wherein said surveillance station includes means to selectively impose a short circuit as an impedance discontinuity to the sharp wave front signal and wherein a capacitance of a value to present an open circuit to said low frequency circuit and a low impedance to said sharp wave front signal is connected in series with said means to impose a short circuit.
 10. A combination according to claim 1 including means at said primary station responsive to a reflection of said signal for actuating said means for issuing the sharp wave front signal to issue such a signal.
 11. A combination according to claim 1 wherein said primary station includes a clocking oscillator; a gate control coupled to said coaxial cable and responsive following issuance of a signal from said signal issuing means and until a reflection of said signal is received from said cable; a gate passing clocking pluses from said oscillator when said gate control is responsive; and a clock pulse counter for counting pulses from said gate.
 12. A combination according to claim 11 including means issuing an actuating signal to said means for issuing a sharp wave front signal; a delay coupled to said means issuing the actuating signal; and inhibiting means for said gate control which are rendered ineffective by a signal from said delay coupled to said delay.
 13. A combination according to claim 1 wherein said primary station includes a phase sensor coupled to said coaxial cable; and means rendering said phase sensor responsive to reflections of the sharp wave front signal following the issuance of the sharp wave front signal by said issuing means.
 14. A combination according to claim 3 including means for ascertaining the phase of the reflected sharp wave front signal.
 15. A combination according to claim 1 wherein said indicator is coupled to said primary station by said coaxial cable and is remote from said station.
 16. A combination according to claim 1 wherein said indicator is a plurality of indicating means each coupled to said primary station by said coaxial cable and spaced from said primary station and each other.
 17. A combination according to claim 16 wherein said sensing means includes means for ascertaining the time between the issuance of the sharp wave front signal and the reflection of said signal; wherein said primary station includes an indicator control responsive to predetermined reflection time intervals defined by said ascertaining means to issue predetermined different coded signals according to said reflection time intervals; and wherein each of a plurality of said indicating means have a decoding actuating means responsive to given predetermined coded signals from said indicator control. 